During the manufacture of a fluorescent lamp, as well as other types of arc discharge lamps, a quantity of elemental mercury is sealed within the lamp envelope. It is known that in operation some of the elemental mercury contained in these lamps can be converted to a mercuric oxide or a mercury salt. This is true in fluorescent lamps in particular. In such lamps most of this mercury adheres to the phosphor coating deposited upon the inside wall of the lamp envelope, leaving only a small portion of the mercury in the form of mercury vapor. After the alkaline earth metal oxides coating the lamp electrodes are volatilized, the oxides decompose in the discharge space, and the freed oxygen converts some of this elemental mercury to a salt or compound such as the above-mentioned mercuric oxide (HgO) which is water soluble.
There is a growing concern that a waste stream resulting from the disposal of arc discharge lamps such as fluorescent lamps may leach excessive amounts of soluble mercury into the environment. One method of measuring the amount of soluble mercury, which may leach from the waste stream resulting from the disposal of fluorescent lamps, is described in the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987–26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. According to the procedure, the lamp being tested is pulverized into granules having a surface area per gram of material equal to or greater than 3.1 cm2 or having a particle size smaller than 1 cm in its narrowest dimension. Following pulverization, the granules are subjected to a sodium acetate buffer solution having a pH of approximately 4.93 and having a weight twenty times the weight of the granules.
At the present time, the Environmental Protection Agency defines a maximum concentration level for mercury at 0.2 milligram leachable mercury per liter extract fluid when the TCLP is applied. According to present standards, a fluorescent lamp is considered nonleachable, and thus available for conventional landfill deposition, when less than 0.2 milligram per liter of leachable mercury results from a TCLP extraction.
Various methods have been proposed which attempt to treat or process burned-out discharge lamps or scrap lamp exhaust tubing containing mercury in order to reclaim the mercury and thereby reduce the amount of mercury-contaminated scrap. These methods are summarized as background in U.S. Pat. Nos. 5,229,686 and 5,229,687, which describe methods by which to render a mercury vapor lamp nonleaching upon disposal without the use of expensive treatment processes to reclaim the mercury. The method of U.S. Pat. No. 5,229,686 employs a chemical agent, enclosed within the lamp, suitable for chemically combining a substantial portion of the soluble mercury as a sparingly soluble salt when the lamp is pulverized as a result of disposal. The method of U.S. Pat. No. 5,229,687 employs a chemical agent, enclosed within the lamp, suitable for electrochemically reducing a substantial portion of the soluble mercury to elemental mercury, again when the lamp is pulverized during disposal. Preferably, this chemical agent is an element which has an electrode potential for oxidation reactions higher than mercury but which is not sufficiently active to displace hydrogen from acidic aqueous solutions. In a preferred embodiment, the chemical agent is sealed within an enclosure (e.g., glass), which is rupturable upon pulverization of the lamp. In another embodiment, the chemical agent is mixed with the basing cement used to secure the lamp bases to the glass envelope. The chemical agent acts to reduce soluble mercury produced during lamp operation to elemental mercury, which is not leachable as measured by the TCLP.
The chemical agent used in '687 may be used in various forms, e.g., as a powder, dust, wire mesh, or metallic foil. The amount or size of the chemical agent is directly related to the surface area and surface condition, finely divided metallic powders being preferred over a solid mass because of their relatively large effective surface areas. Because of their availability and inexpensive cost, iron and copper, in the form of a powder or dust, are preferred. The amount of chemical agent present should be sufficient to electrochemically reduce the amount of soluble mercury within the lamp which is leached at the time of disposal to less than 0.2 milligram per liter of an aqueous acid solution such as a sodium acetate buffer solution as prescribed in the TCLP.
However, there are several disadvantages to the methods described in U.S. Pat. No. 5,229,686 and '687. In regard to '686, the quantity of chemical agent required to chemically combine nearly all of the mercury within a fluorescent lamp may be so large as to be inconvenient or impossible to contain within a standard lamp envelope. In regard to '687, the metallic copper or iron reduces the amount of leachable mercury via a surface redox reaction between adsorbed mercury ions and zero-valent metal atoms. In order for this reaction to occur, the dissolved ionic mercury must first find its way to and become adsorbed upon the metal surface. Thus, the effectiveness of a metallic element as a means of reducing leachable mercury will ultimately be limited by the rates at which mercury ions diffuse to the metal surface and become adsorbed thereon. A means of reducing leachable mercury that did not depend upon the chance contact between dissolved mercury ions and a metal surface followed by the adsorption of the mercury upon that surface would be likely to be more efficient and, therefore, preferable.
It may also be difficult or impossible to incorporate a sufficiently large quantity of a finely divided metal within a fluorescent lamp, the more so the smaller or more compact the lamp. In a small lamp, the only convenient way to introduce the metal may be as a component of the basing cement. However, the electrical conductivity of the metal may prevent its incorporation into the basing cement since the cement may easily come into contact with internal electrical leads. On the other hand, electrically insulating materials might easily be added to the basing cement in addition to or in place of the normal CaCO3 cement filler without risk of creating electrical short circuits within the lamp.
In U.S. Pat. No. 5,736,813, it is disclosed that “the formation of leachable mercury upon disposal or during TCLP testing of mercury vapor discharge lamps is substantially prevented by incorporation of a pH control agent in the lamp structure or in the test solution to provide a pH of about 5.5 to 6.5.” A low pressure mercury discharge lamp is claimed which includes about 5–15 grams of a pH control agent (generally a water-soluble base) which, it is suggested, is sufficient to substantially prevent formation of ferric and cupric compounds which oxidize elemental mercury to a soluble form. The primary disadvantage of this method of reducing mercury leaching is that it may be difficult or, depending upon the lamp type, practically impossible to package the relatively large amounts of the required pH control agent (5–15 grams) within the structure of a typical mercury vapor lamp.
Recently, an improved mercury vapor discharge lamp was described in which an effective amount of a nonmetallic copper-containing compound which, when the lamp is pulverized to granules and subjected to a suitable aqueous acid solution, dissolves in the acid solution, resulting in a concentration of extracted mercury less than 0.2 mg per liter of solution. The effective amount of soluble copper is relatively small (between 0.1 and 4 mg per gram of total lamp weight, depending upon lamp type and size, total mercury loading, etc.). However, copper in the environment, although relatively harmless, may be toxic to certain marine invertebrates. In order to eliminate the possibility of damage to ecological systems, the EPA has placed a limit of 25 mg/L for copper levels in discharges from nonferrous operations to lakes and streams. It is desirable, therefore, to minimize the amount of soluble copper, which is effective with respect to the control of mercury leaching. Further, the smaller the quantity of nonmetallic copper-containing compound, the more easily it will be to incorporate within the lamp.
The use of so-called noble metals and metal salts has also been suggested for the control of mercury leaching in fluorescent lamps. U.S. Pat. No. 6,515,421 describes a method and apparatus for preventing the formation of leachable mercury in mercury arc vapor discharge lamps, which comprises coating at least one of the metallic components of the lamp with at least one noble metal coating (typically silver or palladium). A method and apparatus for preventing the formation of leachable mercury in mercury arc vapor discharge lamps which comprises providing in the lamp structure an effective amount of a silver salt, gold salt, or combination thereof, is described in U.S. Published Patent Application No. 2002/0190646 A1. While these methods may be effective for the control of leachable mercury, they are generally not practical due to the relatively high costs of the noble metals and metal salts. However, the use of such noble metals or metal salts might become practical if a relatively inexpensive means were found to substantially reduce the amounts of these substances which are required to effectively reduce or control mercury leaching.